1. Field of the Invention
The present invention pertains generally to the field of wireless communication devices and, more particularly, to methods and apparatus for controlling power amplifier quiescent current in wireless communication devices.
2. Background
Driven mainly by consumer markets, there is an ongoing demand for increased battery talk and standby time in wireless communication devices--i.e., between battery recharging periods. As such, the power consumption of every circuit in the communication devices is preferably designed to perform its function while consuming the minimum amount of battery power. Optimal performance is further advantageous in that it reduces the amount of heat generated by the communication device.
The use of radio frequency (RF) power transistors as amplifiers in wireless communication devices is well known. With the considerable recent growth in the demand for wireless services, such as personal communication services (PCS), the operating frequency of wireless networks has increased dramatically and is now well into the gigahertz (GHz) range. Achieving consistent performance characteristics in mass-produced amplifier transistor devices, however, is difficult.
FIG. 1 illustrates a typical power amplifier configuration employed in a wireless transmitter 10. An RF input signal to be transmitted, RF.sub.IN, is passed through a transmission band pass filter 14 and then input into a power amplifier 16. The power amplifier 16 also receives an input supply voltage V.sub.S 18 and a reference voltage V.sub.REF 20. In particular, the reference voltage V.sub.REF 20 controls a biasing circuit (not shown) of the power amplifier 16, which, in turn, controls the amplifier DC operating point. Once amplified, the input signal RF.sub.IN is passed through an output impedance matching network 22 and duplexer 24 for transmission via an antenna 26.
The overall efficiency, .eta., of a transmitter to produce the required RF output power in relation to the DC input (i.e., battery) power is: EQU .eta.=(P.sub.OUT.sbsb.--.sub.RF /P.sub.IN.sbsb.--.sub.DC)*100%,
where P.sub.OUT.sbsb.--.sub.RF is the RF power output of the transmitter and P.sub.IN.sbsb.--.sub.DC is the DC input power.
The DC input power is calculated as: EQU P.sub.IN.sbsb.--.sub.DC =V.sub.IN.sbsb.--.sub.DC *I.sub.IN.sbsb.--.sub.DC,
where: V.sub.IN.sbsb.--.sub.DC is the voltage and I.sub.IN.sbsb.--.sub.DC is the current, respectively, of the input voltage supply. Notably, the input voltage : V.sub.IN.sbsb.--.sub.DC may be directly from a battery source, or may be stepped up or down via a controller circuit.
Thus, the transmitter efficiency can be defined as: EQU .eta.=(P.sub.OUT.sbsb.--.sub.RF /(V.sub.IN.sbsb.--.sub.DC *I.sub.IN.sbsb.--.sub.DC))*100%.
For communication devices that are designed to operate in digital wireless systems in which the transmitter output power extends over a relatively large dynamic range, it is of critical importance to incorporate a design that has the highest efficiency over the wide output power range. Power amplifiers used in the linear modulation transmitter schemes for these systems must further exhibit linear performance to meet Adjacent Channel Power (ACP) requirements. In particular, high power amplifier linearity results in better ACP performance, --i.e., low spectral re-growth. Conversely, as the power amplifier linearity decreases, the spectral re-growth increases, resulting in poorer ACP performance.
At the upper output power range of a power amplifier, higher DC input power is generally required for higher linearity at a specific output power. However, higher linearity is generally at the expense of lower efficiency. In practice, it is a difficult challenge to meet ACP and other performance requirements, while minimizing the DC input power to the power amplifier.
In general, the operating parameters of a power amplifier are set in part by the quiescent point and load impedance of each individual amplification stage. The quiescent point is defined by the operating voltage and quiescent current for the particular stage. The quiescent current, I.sub.IN.sbsb.--.sub.DC q, also referred to as "idle current", is the DC input current drawn by the power amplifier in the absence of an RF input signal. In other words, with no RF input signal, the DC input current I.sub.IN.sbsb.--.sub.DC to the power amplifier equals the sum of the quiescent currents of the individual stages. For higher output power, the quiescent current comprises a smaller portion of the DC input current, and for lower output power, a much greater proportion.
It is desirable to minimize the power amplifier quiescent current in order to achieve overall higher transmitter efficiency. Minimizing quiescent current has the most impact on DC input current, and thus battery talk and standby time, when the power amplifier is operated at relatively low output power. In particular, as the amplifier output power is decreased, the DC input current asymptotically approaches the quiescent current. The DC input current essentially equals the quiescent current at a specific RF output power, P.sub.OUT.sbsb.--.sub.RF a, where the designation "a" indicates the power amplifier is operating in class A operation for P.sub.OUT.sbsb.--.sub.RF.sbsb.-- .ltoreq.P.sub.OUT.sbsb.--.sub.RF a. For P.sub.OUT.sbsb.--.sub.RF.sbsb.-- .ltoreq.P.sub.OUT.sbsb.--.sub.RF a, P.sub.IN.sbsb.--.sub.DC =I.sub.IN.sbsb.--.sub.DC q*V.sub.S.
In digital wireless systems where the average power amplifier output power during a call is substantially less than the maximum rated output power of the communication device, it is desirable to minimize the amplifier quiescent current I.sub.IN.sbsb.--.sub.DC q, since it is a major factor in determining battery talk and standby time.
Due to the natural variables of each amplifier in a large scale production of power amplifiers, the respective amplifier quiescent currents will exhibit a broad distribution of values with respect to a fixed biasing control reference voltage. As such, the reference voltage value must be set to a sufficiently high value such that the lowest quiescent current values in the distribution range are still adequate for meeting the minimum power amplifier performance requirements. Of course, the resultant mean battery talk time of communication devices will be lower than would be possible if the quiescent current was optimized on an amplifier by amplifier basis.
Toward this end, U.S. Pat. No. 5,311,143 ("the '143 patent") assigned to Motorola discloses a method for adjusting the power amplifier quiescent current as a function of the total current draw and/or detected RF in/out power. According to this approach, the amplifier quiescent current is adjusted dynamically under presence of an RF input signal, which is somewhat complex and relatively expensive to employ in a very cost sensitive product, as are wireless handsets.
Thus, it would be desirable to provide a relatively simple, economic and reliable means for controlling the power amplifier quiescent current in wireless transmitters.